Polymer wrapped carbon nanotube near-infrared photovoltaic devices

ABSTRACT

A photovoltaic device includes a photoactive region disposed between and electrically connected to two electrodes where the photoactive region includes photoactive polymer-wrapped carbon nanotubes that create excitons upon absorption of light in the range of about 400 nm to 1400 nm.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit under 35 U.S.C. §119(e) of U.S.Provisional Application Ser. No. 61/049,594, filed May 1, 2008, and U.S.Provisional Application Ser. No. 61/110,220, filed Oct. 31, 2008, thedisclosures of which are incorporated herein in their entirety.

GOVERNMENT RIGHTS

This invention was made with Government support from the United StatesAir Force Office of Sponsored Research, Grant No. FA9550-07-1-0364, andDAAB07-01-D-G602, Army Night Vision and Electronic Sensors Directorate(CECOM). The United States Government has certain rights to thisinvention.

FIELD OF THE INVENTION

The present disclosure is related to the field of organicsemiconductors, carbon nanotubes, infrared photodetectors andphotovoltaic cells.

BACKGROUND

Optoelectronic devices rely on the optical and electronic properties ofmaterials to either produce or detect electromagnetic radiationelectronically or to generate electricity from ambient electromagneticradiation.

Photosensitive optoelectronic devices convert electromagnetic radiationinto an electrical signal or electricity. Solar cells, also calledphotovoltaic (“PV”) devices, are a type of photosensitive optoelectronicdevices that are specifically used to generate electrical power.Photoconductor cells are a type of photosensitive optoelectronic devicethat are used in conjunction with signal detection circuitry whichmonitors the resistance of the device to detect changes due to absorbedlight. Photodetectors, which may receive an applied bias voltage, are atype of photosensitive optoelectronic device that are used inconjunction with current detecting circuits which measures the currentgenerated when the photodetector is exposed to electromagneticradiation.

These three classes of photosensitive optoelectronic devices maybedistinguished according to whether a rectifying junction as definedbelow is present and also according to whether the device is operatedwith an external applied voltage, also known as a bias or bias voltage.A photoconductor cell does not have a rectifying junction and isnormally operated with a bias. A PV device has at least one rectifyingjunction and is operated with no bias. A photodetector may have arectifying junction and is usually but not always operated with a bias.

When electromagnetic radiation of an appropriate energy is incident uponan organic semiconductor material, a photon can be absorbed to producean excited molecular state. In organic photoconductive materials, theexcited molecular state is generally believed to be an “exciton,” i.e.,an electron-hole pair in a bound state which is transported as aquasi-particle. An exciton can have an appreciable life-time beforegeminate recombination (“quenching”), which refers to the originalelectron and hole recombining with each other (as opposed torecombination with holes or electrons from other pairs). To produce aphotocurrent, the electron-hole forming the exciton is typicallyseparated at a rectifying junction.

In the case of photosensitive devices, the rectifying junction isreferred to as a photovoltaic heterojunction. Types of organicphotovoltaic heterojunctions include a donor-acceptor heterojunctionformed at an interface of a donor material and an acceptor material, anda Schottky-barrier heterojunction formed at the interface of aphotoconductive material and a metal.

In the context of organic materials, the terms “donor” and “acceptor”refer to the relative positions of the Highest Occupied MolecularOrbital (“HOMO”) and Lowest Unoccupied Molecular Orbital (“LUMO”) energylevels of two contacting but different organic materials. If the HOMOand LUMO energy levels of one material in contact with another arelower, then that material is an acceptor. If the HOMO and LUMO energylevels of one material in contact with another are higher, then thatmaterial is a donor. It is energetically favorable, in the absence of anexternal bias, for electrons at a donor-acceptor junction to move intothe acceptor material.

As used herein, a first HOMO or LUMO energy level is “greater than” or“higher than” a second HOMO or LUMO energy level if the first energylevel is closer to the vacuum energy level. A higher HOMO energy levelcorresponds to an ionization potential (“IP”) having a smaller absoluteenergy relative to a vacuum level. Similarly, a higher LUMO energy levelcorresponds to an electron affinity (“EA”) having a smaller absoluteenergy relative to vacuum level. On a conventional energy level diagram,with the vacuum level at the top, the LUMO energy level of a material ishigher than the HOMO energy level of the same material.

After absorption of a photon in the material creates an exciton, theexciton dissociates at the rectifying interface. A donor material willtransport the hole, and an acceptor material will transport theelectron.

A significant property in organic semiconductors is carrier mobility.Mobility measures the ease with which a charge carrier can move througha conducting material in response to an electric field. In the contextof organic photosensitive devices, a material that conductspreferentially by electrons due to high electron mobility may bereferred to as an electron transport material. A material that conductspreferentially by holes due to a high hole mobility may be referred toas a hole transport material. A layer that conducts preferentially byelectrons, due to mobility and/or position in the device, may bereferred to as an electron transport layer (“ETL”). A layer thatconducts preferentially by holes, due to mobility and/or position in thedevice, may be referred to as a hole transport layer (“HTL”).Preferably, but not necessarily, an acceptor material is an electrontransport material and a donor material is a hole transport material.

How to pair two organic photoconductive materials to serve as a donorand an acceptor in a photovoltaic heterojunction based upon carriermobilities and relative HOMO and LUMO levels is well known in the art,and is not addressed here.

As used herein, the term “organic” includes polymeric materials as wellas small molecule organic materials that may be used to fabricateorganic opto-electronic devices. “Small molecule” refers to any organicmaterial that is not a polymer, and “small molecules” may actually bequite large. Small molecules may include repeat units in somecircumstances. For example, using a long chain alkyl group as asubstitute does not remove a molecule from the “small molecule” class.Small molecules may also be incorporated into polymers, for example as apendent group on a polymer backbone or as a part of the backbone. Smallmolecules may also serve as the core moiety of a dendrimer, whichconsists of a series of chemical shells built on the core moiety. Thecore moiety of a dendrimer may be a fluorescent or phosphorescent smallmolecule emitter. A dendrimer may be a “small molecule.” In general, asmall molecule has a defined chemical formula with a molecular weightthat is the same from molecule to molecule, whereas a polymer has adefined chemical formula with a molecular weight that may vary frommolecule to molecule. As used herein, “organic” includes metal complexesof hydrocarbyl and heteroatom-substituted hydrocarbyl ligands.

An organic photosensitive device comprises at least one photoactiveregion in which light is absorbed to form an exciton, which maysubsequently dissociate into an electron and a hole. The photoactiveregion will typically comprise a donor-acceptor heterojunction, and is aportion of a photosensitive device that absorbs electromagneticradiation to generate excitons that may dissociate in order to generatean electrical current.

Organic photosensitive devices may incorporate electron blocking layers(EBLs). EBLs are described in U.S. Pat. No. 6,451,415 to Forrest et al.,which is incorporated herein by reference for its disclosure related toEBLs. EBLs (among other things) reduce quenching by preventing excitonsfrom migrating out of the donor and/or acceptor materials. It isgenerally believed that the EBLs derive their exciton blocking propertyfrom having a LUMO-HOMO energy gap substantially larger than that of theadjacent organic semiconductor from which excitons are being blocked.Thus, the confined excitons are prohibited from existing in the EBL dueto energy considerations. While it is desirable for the EBL to blockexcitons, it is not desirable for the EBL to block all charge. However,due to the nature of the adjacent energy levels, an EBL may block onesign of charge carrier. By design, an EBL will exist between two otherlayers, usually an organic photosensitive semiconductor layer and anelectrode or a charge transfer layer. The adjacent electrode or chargetransfer layer will be in context either a cathode or an anode.Therefore, the material for an EBL in a given position in a device willbe chosen so that the desired sign of carrier will not be impeded in itstransport to the electrode or charge transfer layer. Proper energy levelalignment ensures that no barrier to charge transport exists, preventingan increase in series resistance.

It should be appreciated that the exciton blocking nature of a materialis not an intrinsic property of its HOMO-LUMO energy gap. Whether agiven material will act as an exciton blocker depends upon the relativeHOMO and LUMO energy levels of the adjacent organic photosensitivematerial, as well upon the carrier mobility and carrier conductivity ofthe material. Therefore, it is not possible to identify a class ofcompounds in isolation as exciton blockers without regard to the devicecontext in which they may be used. However, one of ordinary skill in theart may identify whether a given material will function as an excitonblocking layer when used with a selected set of materials to constructan organic PV device. Additional background explanation of EBLs can befound in U.S. patent application Ser. No. 11/810,782 of Barry P. Rand etal., published as 2008/0001144 A1 on Jan. 3, 2008, the disclosure ofwhich is incorporated herein by reference, and Peumans et al.,“Efficient photon harvesting at high optical intensities in ultrathinorganic double-heterostructure photovoltaic diodes,” Applied PhysicsLetters 76, 2650-52 (2000).

The terms “electrode” and “contact” are used interchangeably herein torefer to a layer that provides a medium for delivering photo-generatedcurrent to an external circuit or providing a bias current or voltage tothe device. Electrodes may be composed of metals or “metal substitutes.”Herein the term “metal” is used to embrace both materials composed of anelementally pure metal, and also metal alloys which are materialscomposed of two or more elementally pure metals. The term “metalsubstitute” refers to a material that is not a metal within the normaldefinition, but which has the metal-like properties such asconductivity, such as doped wide-bandgap semiconductors, degeneratesemiconductors, conducting oxides, and conductive polymers. Electrodesmay comprise a single layer or multiple layers (a “compound” electrode),may be transparent, semi-transparent, or opaque. Examples of electrodesand electrode materials include those disclosed in U.S. Pat. No.6,352,777 to Bulovic et al., and U.S. Pat. No. 6,420,031, toParthasarathy, et al., each incorporated herein by reference fordisclosure of these respective features. As used herein, a layer is saidto be “transparent” if it transmits at least 50% of the ambientelectromagnetic radiation in a relevant wavelength.

The functional components of organic photosensitive devices are usuallyvery thin and mechanically weak, and therefore the devices are typicallyassembled on the surface of a substrate. The substrate may be anysuitable substrate that provides desired structural properties. Thesubstrate may be flexible or rigid, planar or non-planar. The substratemay be transparent, translucent or opaque. Rigid plastics and glass areexamples of preferred rigid substrate materials. Flexible plastics andmetal foils are examples of preferred flexible substrate materials.

Organic donor and acceptor materials for use in the photoactive regionmay include organometallic compounds, including cyclometallatedorganometallic compounds. The term “organometallic” as used herein is asgenerally understood by one of ordinary skill in the art and as given,for example, in Chapter 13 of “Inorganic Chemistry” (2nd Edition) byGary L. Miessler and Donald A. Tarr, Prentice Hall (1999).

Organic layers may be fabricated using vacuum deposition, spin coating,organic vapor-phase deposition, organic vapor jet deposition, inkjetprinting and other methods known in the art.

Donor and acceptor layers may meet at a bilayer, forming a planarheterojunction. A hybrid or mixed heterojunction comprises a mixture ofdonor and acceptor materials, arranged between layers of donor materialand acceptor material. A bulk heterojunction, in the ideal photocurrentcase, has a single continuous interface between the donor material andthe acceptor material, although multiple interfaces typically exist inactual devices. Mixed and bulk heterojunctions can have multipledonor-acceptor interfaces as a result of having plural domains ofmaterial. Domains that are surrounded by the opposite-type material(e.g., a domain of donor material surrounded by acceptor material) maybe electrically isolated, such that these domains do not contribute tophotocurrent. Other domains may be connected by percolation pathways(continuous photocurrent pathways), such that these other domains maycontribute to photocurrent. The distinction between a mixed and a bulkheterojunction lies in degrees of phase separation between donor andacceptor materials. In a mixed heterojunction, there is very little orno phase separation (the domains are very small, e.g., less than a fewnanometers), whereas in a bulk heterojunction, there is significantphase separation (e.g., forming domains with sizes of a few nanometersto 100 nm). Isolated carbon nanotubes act as individual domains, and ifpresent in sufficient concentrations may give rise to percolationpathways.

Small-molecule mixed heterojunctions may be formed, for example, byco-deposition of the donor and acceptor materials using vacuumdeposition or vapor deposition. Small-molecule bulk heterojunctions maybe formed, for example, by controlled growth, co-deposition withpost-deposition annealing, or solution processing. Polymer mixed or bulkheterojunctions may be formed, for example, by solution processing ofpolymer blends of donor and acceptor materials.

In general, planar heterojunctions have good carrier conduction, butpoor exciton dissociation; a mixed layer has poor carrier conduction andgood exciton dissociation, and a bulk heterojunction has good carrierconduction and good exciton dissociation, but may experience chargebuild-up at the end of the material “cul-de-sacs,” lowering efficiency.Unless otherwise stated, planar, mixed, bulk, and hybrid heterojunctionsmay be used interchangeably as donor-acceptor heterojunctions throughoutthe embodiments disclosed herein.

The photoactive region may be part of a Schottky-barrier heterojunction,in which a photoconductive layer forms a Schottky contact with a metallayer. If the photoconductive layer is an ETL, a high work functionmetal such as gold may be used, whereas if the photoconductive layer isan HTL, a low work function metal such as aluminum, magnesium, or indiummay be used. In a Schottky-barrier cell, a built-in electric fieldassociated with the Schottky barrier pulls the electron and hole in anexciton apart. Generally, this field-assisted exciton dissociation isnot as efficient as the dissociation at a donor-acceptor interface.

The devices may be connected to a resistive load which consumes orstores power. If the device is a photodetector, the device is connectedto a current-detecting circuit which measures the current generated whenthe photodetector is exposed to light, and which may apply a bias to thedevice (as described for example in Published U.S. Patent Application2005-0110007 A1, published May 26, 2005 to Forrest et al.). If therectifying junction is eliminated from the device (e.g., using a singlephotoconductive material as the photoactive region), the resultingstructure may be used as a photoconductor cell, in which case the deviceis connected to a signal detection circuit to monitor changes inresistance across the device due to the absorption of light. Unlessotherwise stated, each of these arrangements and modifications may beused for the devices in each of the drawings and embodiments disclosedherein.

An organic photosensitive optoelectronic device may also comprisetransparent charge transfer layers, electrodes, or charge recombinationzones. A charge transfer layer may be organic or inorganic, and may ormay not be photoconductively active. A charge transfer layer is similarto an electrode, but does not have an electrical connection external tothe device and only delivers charge carriers from one subsection of anoptoelectronic device to the adjacent subsection. A charge recombinationzone is similar to a charge transfer layer, but allows for therecombination of electrons and holes between adjacent subsections of anoptoelectronic device. A charge recombination zone may includesemi-transparent metal or metal substitute recombination centerscomprising nanoclusters, nanoparticles, and/or nanorods, as describedfor example in U.S. Pat. No. 6,657,378 to Forrest et al.; Published U.S.Patent Application 2006-0032529 A1, entitled “Organic PhotosensitiveDevices” by Rand et al., published Feb. 16, 2006; and Published U.S.Patent Application 2006-0027802 A1, entitled “Stacked OrganicPhotosensitive Devices” by Forrest et al., published Feb. 9, 2006; eachincorporated herein by reference for its disclosure of recombinationzone materials and structures. A charge recombination zone may or maynot include a transparent matrix layer in which the recombinationcenters are embedded. A charge transfer layer, electrode, or chargerecombination zone may serve as a cathode and/or an anode of subsectionsof the optoelectronic device. An electrode or charge transfer layer mayserve as a Schottky contact.

For additional background explanation and description of the state ofthe art for organic photosensitive devices, including their generalconstruction, characteristics, materials, and features, U.S. Pat. Nos.6,972,431, 6,657,378 and 6,580,027 to Forrest et al., and U.S. Pat. No.6,352,777 to Bulovic et al., are incorporated herein by reference intheir entireties.

The discovery in 1992 of photoinduced charge transfer between conjugatedpolymers and fullerenes (N. S. Sariciftci et al., Proc. SPLE,1852:297-307 (1993)) has inspired a great deal of research into thepossible use of fullerenes in photovoltaic and photoelectric devices.This led to the fabrication of several photovoltaic systems that employa combination of polymer and fullerenes. It has been found thatfullerenes can be susceptible to photooxidation. The observation ofphotoinduced electron transfer at a multi-wall carbonnanotube-conjugated polymer interface (H. Ago et al., Phys. Rev. B,61:2286 (2000)) has inspired attempts to use carbon nanotubes (CNTs) andin particular single-walled carbon nanotubes (SWNTs) as electronacceptor materials in photovoltaic devices.

The first reported use of CNTs as electron acceptors in abulk-heterojunction photovoltaic cell was a blend of SWNTs withpolythiophenes, in which an increase in photocurrent of two orders ofmagnitude was observed (E. Kymakis, G. A. J. Amaratunga, Appl. Phys,Lett. 80:112 (2002)). In 2005, a photovoltaic effect was observed in anisolated SWNT illuminated with 1.5 μm (0.8 eV) radiation (J. U. Li,Appl. Phys. Lett. 87:073101 (2005)).

Kymakis (E. Kymakis and G. Amaratunga, Rev. Adv. Mat. Sci. 10:300-305(2005,) has described the use of carbon nanotubes as electron acceptorsin a polymeric photovoltaic system based on poly (3-octylthiophene). Inthis system, the nanotubes serve as electron acceptors and electronconductors; the photocurrent declines at CNT concentrations greater thanabout 1%, and the authors concluded that the nanotubes do not contributeto the photocurrent.

Ajayan et al., (U.S. Patent Application Publication No. 2006/0272701)have described the use of SWNTs as the electron-transporting componentin a photovoltaic device, using covalently attached organic dyes as thephoto-responsive component. More recently, Mitra et al., have similarlyemployed SWNTs as the electron-transporting component in a photovoltaicdevice based on C₆₀-organic semiconductor heterojunctions (C. Li. etal., J. Mater. Chem. 17, 2406 (2007); C. Li and S. Mitra, Appl. Phys.Lett. 91, 253112 (2007)). These devices employ SWNTs aselectron-accepting and electron-conducting elements. Previous workershave noted that the metallic SWNTs in these devices provideshort-circuit pathways for the recombination of holes and electrons, andhave speculated that the devices would be more efficient if isolatedsemiconducting SWNTs were employed (E. Kymakis et al., J. Phys. D: Appl.Phys. 39, 1058-1062 (2006); M. Vignali et al.,http://re.jrc.cec.eu.int/solarec/publications/paris_polymer.pdf(undated)). However, the use of semiconducting SWNTs in such designsemploy the SWNTs as electron-accepting and electron-conducting elementsonly, rather than as sources of photogenerated excitons. The existingand proposed devices do not take advantage of the photoelectricproperties of semiconducting SWNTs.

Currently, all synthetic methods for growing SWNTs result inheterogeneous mixtures of SWNTs that vary in their structural parameters(length, diameter, and chiral angle), and consequently have variationsin their electronic and optical properties (e.g., conductivity,electrical band gap, and optical band gap) (M. S. Arnold, A. A. Green,J. F. Hulvat et al., NatureNanotech. 1(1), 60 (2006); M. S. Arnold, S.I. Stupp, and M. C. Hersam, Nano Letters 5 (4), 713 (2005); R. H.Baughman, A. A. Zakhidov, and W. A. de Heer, Science 297 (5582), 787(2002). All reported CNT-based photovoltaic devices reported to dateemploy these mixtures.

Recent advances include fabrication methods for CNT thin films onvarious substrates such as (polyethylene terephthalate (PET), glass,polymethylmethacrylate) (PMMA), and silicon (Y. Zhou, L. Hu, G. Griiner,Appl. Phys. Lett. 88:123109 (2006)). The method combines vacuumfiltration generation of CNT mats with a transfer-printing technique,and allows controlled deposition and patterning of large area, highlyconducting CNT films with high homogeneity. Such films are a potentialalternative to the commonly-used hole-collecting electrode material,indium-tin oxide (ITO), which is expensive and remains incompatible withroll-to-roll fabrication processing.

The properties of carbon nanotubes are influenced mostly by the diameterof the tube and the degree of twist. Both aligned tubes and tubes with atwist can be metallic or semiconducting, depending on whether the energystates in the circumferential direction pass through what is termed aFermi point. At Fermi points, the valence and conduction bands meet,which allows for conduction in the circumferential direction of thetube. Tubes that have the correct combination of diameter and chiralitywill possess a set of Fermi points around the perimeter of their gridstructures throughout the length of the tube. These tubes will showmetallic like conduction. If the diameter and chirality do not generatea set of Fermi points, the tube will exhibit semiconducting behavior (P.Avouris, Chemical Physics, 281: 429-445 (2002)).

In addition to Fermi point matchups, the cylindrical shape and diameterof the tube affects electron transport through the way in which quantumstates exist around the tube perimeter. Small diameter tubes will have ahigh circumferential band gap with a low number of energy statesavailable. As the tube diameter increases, the number of energy statesincreases and the circumferential band gap decreases. In general, theband gap is inversely proportional to the tube diameter.

Furthermore, the wave properties of electrons are such that standingwaves can be set up radially around a carbon nanotube. These standingwaves, the lack of conduction states in small diameter tubes, and themonolayer thickness of the graphite sheet, combine to inhibit electronmotion around the tube perimeter and force electrons to be transportedalong the tube axis.

If a Fermi point matchup is present, however, electron transport canoccur around the tube perimeter, in addition to axial conduction,allowing for increased transport options of the electron and metallicconduction characteristics. As the tube diameter increases, more energystates are allowed around the tube perimeter and this also tends tolower the band gap. Thus, when only axial conduction is allowed, thetube exhibits semiconducting behavior. When both axial andcircumferential conduction are allowed, the tube exhibits metallicconduction.

The power output of existing organic photovoltaic devices is not yetcompetitive with traditional silicon-based photovoltaic devices. Inaddition to being less efficient and like other thin-film approaches,they are susceptible to oxidative degradation when exposed to air, andneed encapsulation. Given the cost and fragility of silicon solar cells,and the promise of easily-fabricated and inexpensive organicequivalents, there remains a need for more efficient and more stableorganic photovoltaic and photoelectric devices. Also, because of organicmaterials' poor sensitivity to IR and near-IR radiation, there remains aneed for organic photovoltaic materials capable of efficiently producingexcitons upon irradiation by IR and NIR radiation.

Semiconducting CNTs, despite their strong near-IR band gap absorption,have only had limited impact as the optically absorptive components ofoptoelectronic devices because of the strong binding energy ofphotogenerated electron-hole pairs.

SUMMARY

The present disclosure provides photoelectric and photovoltaic devicesin which semiconducting carbon nanotubes serve as organicphotoconductive materials, i.e. as the light-harvesting component. Inparticular, the present disclosure describes the use of carbon nanotubesas a material for detection of IR radiation in thin film devicearchitecture. In these devices, semiconducting carbon nanotubes act aselectron donors and separation of the photogenerated charges takes placeat a heterojunction between the semiconducting carbon nanotubes and anorganic semiconductor. Appropriate selection of the diameter and opticalband gaps of the semiconducting carbon nanotubes may be used to vary theresponsivity of the photovoltaic device from the visible to thenear-infrared region of the spectrum. Representative materials, devicearchitectures, and procedures for fabricating the architectures areoutlined herein.

According to an embodiment of the present disclosure, at least one orboth of the organic acceptor and donor layers in the photoactive regionof the photovoltaic device includes carbon nanotubes. The presentdisclosure describes the use of carbon nanotubes as optically activecomponents of large-area optoelectronic devices.

In one embodiment, a photovoltaic device comprises a first electrode, asecond electrode, and a photoactive region disposed between andelectrically connected to the first electrode and the second electrode.The photoactive region further comprises a donor layer formed above thefirst electrode, and an acceptor layer formed above the donor layer suchthat the donor layer and the acceptor layer form a donor-acceptorheterojunction, wherein either the acceptor layer or the donor layercomprises a layer of polymer-wrapped carbon nanotubes.

In another embodiment, a photovoltaic device comprises a firstelectrode, a donor layer formed above the first electrode, a bulkheterojunction formed above the donor layer, wherein the bulkheterojunction comprises polymer-wrapped carbon nanotubes disposedwithin an organic electron acceptor. An acceptor layer is formed abovethe bulk heterojunction, and a second electrode is formed above theacceptor layer. Either the acceptor layer or the donor layer cancomprise a layer of polymer-wrapped carbon nanotubes.

The inventors have discovered that excitons (bound electron-hole pairs)in CNTs can be efficiently dissociated by interfacing CNTs with theelectron acceptor such as evaporated C₆₀. The two fullerene-basedmaterials form a donor-acceptor heterojunction with band/orbital offsetsthat are sufficient to result in electron transfer from the CNTs toevaporated C₆₀. The combination of the visible absorptivity of theorganic semiconductors and the near-IR absorptivity of the CNTs resultsin broadband sensitivity to electromagnetic illumination varying from400-1400 nm in wavelength.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B illustrate the architectures of planar heterojunctionembodiments of the present disclosure.

FIGS. 2A-2D illustrate the architectures of bulk heterojunctionembodiments of the present disclosure.

FIGS. 3A and 3B illustrate the architectures of additional bulkheterojunction embodiments of the present disclosure.

FIG. 4 shows the architecture of a planar heterojunction formed bydepositing a carbon nanotube film onto PTCDA(3,4,9,10-perylene-tetracarboxyl-bis-dianhydride).

FIG. 5 shows the current-voltage curve of the heterojunction in the darkand illuminated with simulated solar near-IR radiation.

FIG. 6 shows the conduction band (CB) and valence band (VB) energies ofCNTs for different absorption wavelengths as well as the necessaryenergies for acceptable donors or acceptors.

FIG. 7 shows the photoluminescence of polymer-wrapped carbon nanotubessuspended in toluene, excited at 650 nm.

FIG. 8 shows photoluminescence intensity of thick films containing PFOand CNTs doctor bladed from toluene solution.

FIG. 9A shows an example of architecture of a polymer-wrapped carbonnanotube/C₆₀ heterojunction diode with a 1:1 ratio of MDMO-PPV tonanotubes, by weight.

FIG. 9B shows the current-voltage characteristics of the device of FIG.9A.

FIG. 9C shows the spectrally resolved photoresponsivity of the device ofFIG. 9A.

FIG. 9D shows the internal quantum efficiency (IQE) of the device ofFIG. 9A and absorptivity of polymer wrapped carbon nanotubes.

FIG. 10A is a schematic energy level diagram for a polymer wrappedcarbon nanotube/C₆₀ heterojunction according to an embodiment of thepresent disclosure.

FIGS. 10B and 10C are schematic energy level diagrams of two controldevices.

FIG. 11A shows another example of an architecture of a polymer-wrappedcarbon nanotube/C₆₀ heterojunction diode with a 1:1 ratio of MDMO-PPV tonanotubes, by weight.

FIG. 11B shows the current-voltage characteristics of the device of FIG.11A.

FIG. 11C shows the spectrally resolved photoresponsivity of the deviceof FIG. 11A.

FIG. 11D shows the internal quantum efficiency (IQE) of the device ofFIG. 11A and absorptivity of polymer wrapped carbon nanotubes.

The features shown in the above referenced drawings are illustratedschematically and are not intended to be drawn to scale nor are theyintended to be shown in precise positional relationship. Like referencenumbers indicate like elements.

DETAILED DESCRIPTION

In the following detailed description of the preferred embodiments,reference is made to the accompanying drawings which form a part hereof,and in which are shown by way of illustration specific embodiments inwhich the invention may be practiced. It is to be understood that otherembodiments may be utilized and structural changes may be made withoutdeparting from the scope of the present invention.

An organic photosensitive optoelectronic device according to anembodiment of the present disclosure can be used, for example, to detectincident electromagnetic radiation particularly electromagneticradiation in the IR and near-IR spectrum or as a solar cell to generatepower. Embodiments of the present invention may comprise an anode, acathode, and a photoactive region between the anode and the cathode,wherein semiconducting polymer-wrapped carbon nanotubes and an organicsemiconductor form a heterojunction within the photoactive region. Thephotoactive region is the portion of the photosensitive device thatabsorbs electromagnetic radiation to generate excitons that maydissociate in order to generate an electrical current. Organicphotosensitive optoelectronic devices may also include at least onetransparent electrode to allow incident radiation to be absorbed by thedevice.

An efficient photosensitive optoelectronic device formed of carbonnanotubes contains compounds with proper energies such that the excitoncreated by absorption of a photon by a carbon nanotube is split into afree electron and a free hole. To efficiently split the exciton, theHOMO of the donor material should be higher in energy (less negative)than the valance band (VB) of the carbon nanotube-based acceptor. Orconversely, the LUMO of the acceptor material must be less than (morenegative) than the conduction band (CB) of the carbon nanotube-baseddonor. The CB and VB energies as well as the necessary energies foracceptable donors or acceptors are shown in FIG. 6 (see R. B. Weisman,et al. NANO LETT 3 (2003)).

As used herein, the term “rectifying” denotes, inter alia, that aninterface has an asymmetric conduction characteristic, i.e., theinterface supports electronic charge transport preferably in onedirection. The term “semiconductor” denotes materials which can conductelectricity when charge carriers are induced by thermal orelectromagnetic excitation. The term “photoconductive” generally relatesto the process in which electromagnetic radiant energy is absorbed andthereby converted to excitation energy of electric charge carriers sothat the carriers can conduct (i.e., transport) electric charge in amaterial. The term “photoconductive material” refers to semiconductormaterials which are utilized for their property of absorbingelectromagnetic radiation to generate electric charge carriers. As usedherein, “top” means furthest away from the substrate, while “bottom”means closest to the substrate. There may be intervening layers (forexample, if a first layer is “on” or “over” a second layer), unless itis specified that the first layer is “in physical contact with” or“directly on” the second layer; however, this does not preclude surfacetreatments (e.g., exposure of the first layer to hydrogen plasma).

Referring to FIGS. 1A and 1B, architectures of two planar heterojunctionembodiments for a photovoltaic device 100A and 100B are disclosed. Thephotovoltaic device 100A comprises a conducting anode layer 110A, anelectron donor layer 120A formed above the anode layer 110A, an electronacceptor layer 130A formed above the donor layer 120A and a conductingcathode layer 150A formed above the electron acceptor layer 130A. Inthis embodiment, a thin film of polymer-wrapped carbon nanotubes formthe electron donor layer 120A. A planar heterojunction is formed betweenthe electron donor layer 120A and the electron acceptor layer 130A. Theelectron donor layer 120A and the electron acceptor layer 130A form thephotoactive region 122A of the device 100A. Preferably, thepolymer-wrapped carbon nanotubes (PW-CNTs) are substantiallysemiconducting polymer-wrapped single-wall carbon nanotubes (PW-SWNTs).Although PW-SWNTs are preferred, polymer-wrapped carbon nanotubesincluding multi-walled carbon nanotubes are within the scope of theinvention disclosed herein. Therefore, in the various embodimentspresented herein, when PW-SWNTs are mentioned in connection with a PVdevice, those embodiments are only examples and other embodiments usingPW-CNTs generally, including polymer-wrapped multi-walled carbonnanotubes, are within the scope of the present disclosure.

When the PW-CNTs are used as the electron donor, suitable organicsemiconductors for forming the electron acceptor layer 130A include, butare not limited to, evaporated C₆₀ having a LUMO of −4.0 eV, [84]PCBM([6,6]-Phenyl C₈₄ butyric acid methyl ester) having a LUMO of −4.1 eV,F16-CuPc having a LUMO of −4.4 eV, PTCBI (3,4,9,10perylenetetracarboxylic bisbenzimidazole) having a LUMO of −4.0 eV,PTCDA (3,4,9,10 perylene-tetracarboxylic dianhydride) having a LUMO of−4.7 eV, or Poly(benzimidazobenzo phenanthroline) having a LUMO of −4.5eV, TCNQ (7,7,8,8-tetracyanoquinodimethane) having a LUMO of 3.9 eV,F4-TCNQ (tetrafluorotetracyanoquinodimethane) having a LUMO of 5.2 eV,and the like.

The organic semiconductor for the electron acceptor layer 130A ispreferably capable of efficiently delivering the electrons to thecathode 150A, or to an electron transport layer. These suitable organicsemiconductors for the electron acceptor layer 130A typically have aLUMO of lower energy than the LUMO of the carbon nanotubes, so thatelectron transfer from the irradiated carbon nanotubes (preferablyPW-SWNTs) is rapid and irreversible.

There are many different organic semiconductors that could form arectifying heterojunction with semiconducting PW-CNTs, which results incharge separation and charge transfer of photogenerated charge and aphotovoltaic effect. For a CNT with an optical band gap of 1 eV and anexciton binding energy of 0.5 eV, the expected HOMO-LUMO or electricalband gap would be 1.5 eV. Assuming a p-type doping and taking 4.6 eV asthe work-function (see calculations of V. Barone, J. E. Peralta, J.Uddin et al., J, Chem. Phys. 124(2) (2006)), the LUMO or conduction bandwould sit at 3.5 eV with reference to vacuum while the HOMO or valenceband would sit at 5.0 eV from vacuum. Thus, such a semiconducting PW-CNTwould form a rectifying heterojunction with evaporated C₆₀ as theelectron acceptor.

As noted above, the exact band energy levels of semiconducting CNTsdepend on their diameter, chiral twist, electrical band gap, opticalband gap, local dielectric environment, and doping. Thus, semiconductingCNTs can serve as either electron accepting material or electrondonating material in a heterojunction with another organicsemiconductor, depending on the structure of the nanotube and theproperties of the organic semiconductor. In addition to small moleculeorganic semiconductors, conducting polymers could be utilized as eitheran electron accepting or electron donating materials as well.

In the embodiment shown in FIG. 1B, the photovoltaic device 100Bcomprises a conducting anode layer 110B, an electron donor layer 120Bformed above the anode layer 110B, an electron acceptor layer 130B, anda conducting cathode layer 150B. In this embodiment, a thin film ofpolymer-wrapped carbon nanotubes form the electron acceptor layer 130B.A planar heterojunction is formed between the electron donor layer 120Band the electron acceptor layer 130B. The electron donor layer 120B andthe electron acceptor layer 130B form the photoactive region 122B of thedevice 100B.

Preferably, the polymer-wrapped carbon nanotubes are substantiallysemiconducting PW-SWNTs. Carbon nanotubes may be single-walled ormulti-walled. Multi-walled nanotubes contain multiple layers of graphitearranged concentrically in a tube. Generally, SWNTs exhibit betterelectrical properties than multi-walled nanotubes. SWNTs commerciallyavailable in bulk quantity are generally manufactured using either ahigh-pressure carbon monoxide (HiPCO®) process (such as HiPCO® nanotubesavailable from Unidym of Menlo Park, Calif., U.S.A.) or an arc-dischargeprocess (such as P3 nanotubes from Carbon Solutions Inc., which arepurified arc-discharge nanotubes with two open ends linked withhydrophilic carboxyl groups).

As used herein, “substantially semiconducting PW-SWNTs” refer topopulations of PW-SWNTs in which at least 80% by weight of the nanotubesare of the semiconducting variety, i.e. non-metallic. It will beappreciated that as the fraction of conducting nanotubes is reduced, thedensity of nanotubes in the photoactive regions of the photodetectingdevices may be increased while maintaining a very low probability ofpercolating conducting paths. Accordingly, in preferred embodiments, atleast 90% of the nanotubes are of the semiconducting variety, and inmore preferred embodiments, at least 95% are of the semiconductingvariety. In the most preferred embodiments, at least 99% of thenanotubes are of the semiconducting variety.

In a typical CNT mixture, one third of CNTs are metallic in nature whilethe remaining two thirds are semiconducting, with optical and electricalband gaps that roughly vary inversely with diameter. This heterogeneitypresents an obstacle to the fabrication of efficient photovoltaic solarcells from as-produced CNT mixtures, due to excitonic quenching andnonrectifying electrical paths associated with the presence of metallicCNTs. Isolated semiconducting carbon nanotube preparations are necessaryto create an efficient organic-semiconductor-semiconducting carbonnanotube heterojunction photovoltaic solar cell. At present, the onlyway to isolate semiconducting SWNTs is via post-synthesis processingmethods.

Currently, a few such processing methods exist for enriching orisolating semiconducting CNTs on the laboratory scale. These methodsinclude “constructive destruction” (P. C. Collins, M. S. Arnold, and P.Avouris, Science 292(5517), 706 (2001)); selective etching of metallicCNTs in monolayer thin films (G. Y. Zhang, P. F. Qi, X. R. Wang et al.,Science 314(5801), 974 (2006)); field-flow fractionation based ondielectrophoresis (H. Q. Peng, N. T. Alvaret, C. Kittrell et al., J.Amer. Chem. Soc. 128(26), 8396 (2006)); and anion exchangechromatography of DNA-wrapped CNTs (M. Zheng, A. Jagota, M. S. Strano etal., Science 302(5650), 1545-1548 (2003)). However, the effectiveness ofmany of these techniques (the proportion of obtained CNTs that aresemiconducting) is limited or unclear, and there are significantdrawbacks to these techniques that render them impractical for producingusable quantities of semiconducting CNTs.

When the PW-CNTs are used as the electron acceptors, suitable organicsemiconductors for forming the electron donor layer 120B include, butare not limited to, BTEM-PPV(Poly(2,5-bis(1,4,7,10-tetraoxaundecyl)-1,4-phenylenevinylene) having aHOMO of −4.97 eV, Poly(3-decyloxythiophene) having a HOMO of −4.5 eV,CuPc (copper phthalocyanine) having a HOMO of 5.3 eV, NPD(4,4′-bis(N-(1-napthyl)phenylamino)biphenyl) having a HOMO of 5.4 eV,pentacene having a HOMO of 5.0 eV, tetracene having a HOMO of 5.4 eV,and the like. The organic semiconductor for the electron donor layer120B should be capable of efficiently delivering the holes to the anode110B, or to a hole transport layer. The suitable organic semiconductorsfor the electron donor layer 120B are preferably those having a HOMO ofhigher energy than the HOMO of the carbon nanotubes, so that holetransport from (electron transfer to) the irradiated CNTs is rapid andirreversible.

In both embodiments 100A and 100B, an optional exciton blocking layer140A, 140B can be provided between the photoactive regions 122A, 122Band the cathode layers 150A, 150B, respectively. Additionally, anoptional exciton blocking layer 115A, 115B can be provided between thephotoactive regions 112A, 122B and the anode layers 110A, 110B,respectively. An anode-smoothing layer may also be situated between theanode and the donor. Anode-smoothing layers are described in U.S. Pat.No. 6,657,378 to Forrest et al., incorporated herein by reference forits disclosure related to this feature

Polymer Wrapping

The carbon nanotubes, as produced, are highly agglomerated and bundled.To get efficient optical absorption and exciton splitting and to preventexciton quenching, the tubes must be debundled. This is done through aknown polymer wrapping process. The carbon nanotubes are placed into asolution of polymer and the appropriate solvent and the carbon nanotubesare separated using a high-power horn sonicator (cell dismembrator). Ifan appropriate polymer is used (various poly-thiophene polymer,poly-phenylenevinylene polymer, and poly-fluorene polymer derivatives,amongst others) the polymer will wrap with soluble side-groups on thepolymer creating a carbon nanotube-polymer complex that is soluble. Themain purpose of the polymer wrapping is to suspend individual nanotubes.A polymer that will form a donor-acceptor heterojunction with the carbonnanotubes so as to facilitate the splitting of the exciton is notnecessary for the wrapping material, as long as another material ispresent in the device that can form a donor-acceptor heterojunction withthe PW-CNTs.

After the carbon nanotubes are polymer wrapped and solubilized, aphotovoltaic device can be made by incorporating them into a donor oracceptor molecule or polymer, if needed, and casting into a device. Tocreate a photovoltaic device with sufficient carbon nanotubes to absorban appreciable amount of light may require a high enough concentrationof carbon nanotubes that create a percolating network (greater than ˜1%by weight carbon nanotubes in the film). This means that any metalliccarbon nanotubes that touch a semiconducting carbon nanotube will act asan exciton quenching center and will drastically reduce the efficiencyof the photovoltaic device. Also, the metallic carbon nanotubes couldact to short out the device by creating metal fibers that traverse theentire cell thickness leading to a reduced shunt resistance. To avoidthis phenomenon, the carbon nanotubes are sorted by a method, such asdensity gradient ultracentrifugation, so that nearly all of the metalliccarbon nanotubes are removed and the exciton quenching is significantlyreduced.

In some embodiments, photoactive polymers such aspoly[2-methoxy-5-(3′,7′-dimethyloctyloxy)-1,4-phenylenevinylene](MDMO-PPV), poly[2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylene vinylene](MEH-PPV) and poly(9,9-dioctylfluorenyl-2,7-diyl) (PFO) can be used towrap and solubilize the carbon nanotubes. In such embodiments, thephotoactive polymers wrapping the CNTs absorb light creating excitonsthat are separated at the wrapping polymer-to-carbon nanotube orwrapping polymer-to-organic (donor or acceptor) interface independent ofthe CNTs.

Other examples of photoactive polymers that can be used to wrap theSWNTs are PFO: Poly(9,9-dioctylfluorenyl-2,7-diyl) and polymers with thesame backbone and different solublizing groups such as orPFH—Poly(9,9-dihexylfluorenyl-2,7-diyl) orPoly[9,9-di-(2-ethylhexyl)-fluorenyl-2,7-diyl]. Another extension isthat sometimes copolymers can be used (alternating between PFO andanother monomer such asPoly[(9,9-dioctylfluorenyl-2,7-diyl)-alt-(1,4-vinylenephenylene)],Poly[(9,9-dioctylfluorenyl-2,7-diyl)-alt-(vinyleneanthracene)], orPoly[9,9-dioctylfluorenyl-2,7-diyl)-co-1,4-benzo-{2,1′-3}-thiadiazole)].

Another example is phenylenevinylene based polymers: such asMDMO-PPV—Poly[2-methoxy-5-(3,7-dimethyl-octyloxy)-1,4-phenylenevinylene)or MEH-PPV—Poly[2-methoxy-5-(2-ethyl-hexyloxy)-1,4-phenylenevinylene]and polymers with the same backbone and different solublizing groupssuch as poly[2,5-bis(3,7-dimethyloctyloxy)-1,4-phenylene-vinylene].Sometimes backbone alternates such as copolymers can be used(alternating between PFO and another monomer such asPoly[(9,9-dioctylfluorenyl-2,7-diyl)-alt-(1,4-vinylenephenylene)],Poly[(9,9-dioctylfluorenyl-2,7-diyl)-alt-(vinyleneanthracene)], orPoly[9,9-dioctylfluorenyl-2,7-diyl)-co-1,4-benzo-{2,1′-3}-thiadiazole)].

Thiophene based polymers such as P3HT or those using other solublizinggroups could be used: P3BT—poly(3-butyl-thiophene-2,5-diyl);P3HT—poly(3-hexyl-thiophene-2,5-diyl);P3OT—poly(3-octal-thiophene-2,5-diyl);P3DT—poly(3-decyl-thiophene-2,5-diyl), etc. Sometimes backbonealternates such as copolymers can be used (alternating between PFO andanother monomer such asPoly[(9,9-dioctylfluorenyl-2,7-diyl)-alt-(1,4-vinylenephenylene)],Poly[(9,9-dioctylfluorenyl-2,7-diyl)-alt-(vinyleneanthracene)], orPoly[9,9-dioctylfluorenyl-2,7-diyl)-co-1,4-benzo-{2,1′-3}-thiadiazole)].Other conducting polymer backbones such as PPE polymers:Poly(2,5-dioctyl-1,4-phenylene), with the same additions and low-bandgappolymers such aspoly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b]-dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)](PCPDTBT) are also suitable. Variations might be made to the backbone oralternating unit.

FIG. 7 shows the photoluminescence of polymer-wrapped carbon nanotubessuspended in toluene, excited at 650 nm, showing that the wrapping workswell with MDMO-PPV, and that for certain carbon nanotube chiralities,PFO gives much brighter photoluminescence. The signal intensity is anindicator of the amount of carbon nanotubes that are individuallydispersed. Aggregated and bundled nanotubes show no photoluminescencesignal because of quenching due to metallic nanotubes in contact withsemiconducting nanotubes. This shows that MDMO is comparable to MEH-PPVin its ability to solubilize carbon nanotubes in toluene. This suggeststhat minor changes to the side-groups do not significantly alter thewrapping efficiency and that many similar polymers may be used.

MDMO-PPV wrapping imparts solubility to the carbon nanotubes in organicsolvents thus facilitating their solution-based processing. The MDMO-PPVwas also expected to effectively isolate the carbon nanotubes fromone-another, thus minimizing the direct electronic coupling between theoptically active semiconducting nanotubes and any quenching metallicnanotubes.

In the inventors' experiments, the carbon nanotubes were first wrappedby a semiconducting polymer,poly[2-methoxy-5-(3′,7′-dimethyloctyloxy)-1,4-phenylenevinylene](MDMO-PPV). High pressure carbon monoxide (HiPCO) grown nanotubes thatvaried from 0.7-1.1 nm in diameter were utilized because to insurespectral responsivity extending to 1400 nm in wavelength. The MDMO-PPVwrapped nonotubes were purified by centrifugation in order to removebundles of nanotubes and insoluble material. The polymer-nanotubemixture was spread over a hot indium tin oxide (ITO)-coated glasssubstrate via doctor-blading in an inert nitrogen atmosphere. A thinfilm of evaporated C₆₀ was thermally evaporated on top of thepolymer-nanotube mixture to form the donor-acceptor interface, followedby a 10 nm buffer layer of 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline(BCP), and then the Ag cathode.

FIG. 8 shows the inventors' experimental results on [84] PCBM thatverifies that the theoretical quenching due to the energy levelsdescribed in FIG. 6 is relatively accurate. The data shown in FIG. 8were taken from a thick film doctor bladed from toluene solutions. Thefilm containing PFO and CNTs is a reference for the effect of addingfullerene additives. The addition of [84] PCBM completely quenches allnanotube luminescence in the wavelength probed (950 to 1350 nm whenexcited at 650 nm), indicating that the [84] PCBM energy level is deeperthan the LUMO minus the exciton binding energy, and results in efficientsplitting of excitons in the CNTs. This indicates that [84] PCBM is aneffective material for splitting excitons on the CNTs. Therefore, [84]PCBM is an effective electron acceptor material to be used inconjunction with the substantially semiconducting PW-CNTs used as thephotoconductive material.

Referring to FIGS. 9A-9D, the current-voltage characteristics (FIG. 9B)and spectrally resolved photoresponsivity (FIG. 9C) of a PW-CNT/C₆₀heterojunction diode with a 1:1 ratio of MDMO-PPV to nanotubes, byweight, shown in FIG. 9A are depicted. The thickness of the layers inthe device are provided in FIG. 9A. The diodes have rectification ratiosof more than four orders of magnitude under dark conditions (FIG. 9B).The forward bias current-voltage characteristics follow the Shockleydiode equation with a diode ideality factor of 2.5 and a seriesresistance of 120Ω. Referring to FIG. 9C, the near-IR responsivity ofthe diodes at 0V bias (plot line 92) and −0.5V bias (plot line 93) iscompared with the absorption spectrum of isolated semiconducting CNTs inan aqueous surfactant solution of sodium cholate in de-ionized H₂O (plotline 94). The photoresponsivity is red-shifted by about 40 meV from thesolution absorption spectrum but follows the same shape. The peakphotoresponsivity of the devices at 1155 nm at a bias of 0V (plot line92) and −0.5V (plot line 93) was about 10 and 17 mA/W, respectively. Incomparison, the responsivity of control devices without carbon nanotubeswas immeasurable in the near-infrared (<0.1 μA/W).

FIG. 9D is a plot of the internal quantum efficiency (IQE) of the deviceof FIG. 9A. (See solid line 95). IQE is the ratio of thephotoresponsivity of the device under −0.5V bias (plot line 93 in FIG.9C) to the near-IR absorbtivity of the PW-CNTs (dashed line 97 in FIG.9D). The near-IR absorptivity of the PW-CNTs (dashed line 97) wasquantified by measuring the spectrally resolved reflectivity of thedevice and then subtracting the absorption due the ITO. As shown, thepeak IQE in the near-IR exceeds about 20% over the broad spectral rangeof 1000 to 1350 nm. The substantially large IQE indicates that there isa favorable mechanism for exciton dissociation in the devices.

To test the hypothesis that the active/dissociating interface in ourdevice was the PW-CNT/C₆₀ interface, the inventors fabricated twocontrol device architectures in which the PW-CNT/C₆₀ interface wasbroken. The schematic energy diagrams of these structures are shown inFIGS. 10B and 10C. In the first control device architecture shown inFIG. 10B, a 40 nm layer of sub phthalocynine (SubPc) 14 was insertedbetween the PW-CNT layer 11 and the evaporated C₆₀ 13 to break thePW-CNT/C₆₀ interface. The HOMO and LUMO energies of SubPc are similar tothose in the MDMO-PPV. The second control device architecture wasfabricated to test for exciton dissociation within the MDMO-PPV wrappedcarbon nanotube layer, itself. In this architecture, the evaporated C₆₀layer 13 was removed and a buffer layer of PFO 15 was inserted as a holetransport layer between the ITO and the PW-CNT layer 11 to prevent thenanotubes from directly bridging the anode and cathode. Thecorresponding energy diagram for the second control device architectureis shown in FIG. 10C.

Near-IR responsivity originating from the PW-CNTs was not observed ineither control device (responsivity <0.1 μA/W) indicating that there wasan insufficient driving force for exciton dissociation at thePW-CNT/ITO, PW-CNT/SubPc, and PW-CNT/MDMO-PPV interfaces. Measurablephotocurrent in response to near-infrared illumination was only observedwhen the PW-CNT/C₆₀ interface was left intact.

FIGS. 10A-10C show the expected energy alignments between the variousorganic semiconductors and an (8,4) semiconducting PW-CNT. An (8,4)nanotube has a diameter of 0.84 nm and an expected optical band gap inthe polymer matrix at 1155 nm. The electron affinity (EA) and ionizationpotential (IP) of the nanotube were determined from first principlescalculations of the nanotube electronic band structure by Spataru etal., Excitonic effects and optical spectra of single-walled carbonnanotubes, Physical Review Letters 92(7) (2004), and Perebeinos et al.,Scaling of excitons in carbon nanotubes, Physical Review Letters 92(25)(2004), and of the work function by Barone et al., Screened exchangehybrid density-functional study of the work function of pristine anddoped single-walled carbon nanotubes, Journal of Chemical Physics 124(2)(2006). First principles calculations of the electronic structure ofevaporated C₆₀ shows that an offset of 0.2 eV between the EA of thenanotube and the LUMO of evaporated C₆₀ is expected (see FIGS. 10A and10B). For comparison, the binding energy of an exciton in an (8,4)semiconducting PW-CNT with a relative dielectric permittivity of 3.5 isexpected to be 0.1 eV. Therefore, the energy offset should be sufficientto result in exciton dissociation and charge transfer from the carbonnanotubes to evaporated C₆₀.

In contrast, exciton dissociation at the interface of the MDMO-PPV andthe carbon nanotubes should not be expected. Rather, it is expected thatthese two materials will form a straddling type I heterojunction whereboth the IP and EA of the carbon nanotubes lie within the HOMO and LUMOlevels of MDMO-PPV. The existence of a straddling type I heterojunctionbetween MDMO-PPV and carbon nanotubes has been experimentally supportedby photoluminescence spectroscopy in which strong photoluminescence frompolymer-wrapped semiconducting carbon nanotubes has been observed at theoptical band gap of the nanotubes in response to direct optical excitonof the polymer absorption band.

The planar heterojunctions described above can be formed by depositing athin film of organic semiconductor directly on top of a percolatingnetwork of CNTs. An electron transporting and/or exciton blocking layer140A may optionally be added, followed by deposition of the cathodelayer 150A, to produce the photovoltaic device architecture 100A shownin FIG. 1A. Alternatively, a thin film of CNTs can be stamped onto athin film of an organic donor material deposited on an anode. Depositionof the optional electron transporting and/or exciton blocking layer140B, followed by the cathode layer 150B, produces the photovoltaicdevice architecture 100B shown in FIG. 1B.

Thin films of percolating networks of CNTs can be prepared by directgrowth, by vacuum filtration through porous membranes, spray-baseddeposition strategies, spin-coating, layer-by-layer depositionapproaches, dielectrophoresis, and evaporation.

FIGS. 2A-2B show two hybrid planar-bulk heterojunction embodiments for aphotovoltaic device 200A and 200B. Referring to FIG. 2A, thephotovoltaic device 200A comprises a conducting anode layer 210A and abulk heterojunction layer 220A comprising polymer wrapped carbonnanotubes disposed within an organic electron donor material formedabove the anode layer 210A. An electron acceptor layer 230A is formedabove the bulk heterojunction layer 220A and a conducting cathode layer250A formed above the electron acceptor layer 230A. The bulkheterojunction layer 220A and the electron acceptor layer 230A form thephotoactive region 222A of the device 200A. Alternatively, the layers220A and 230A can be configured such that the polymer wrapped carbonnanotubes in the layer 220A and the electron acceptor material of thelayer 230A form a bulk heterojuction.

The suitable organic semiconductor donor materials for forming the bulkheterojunction layer 220A include, but are not limited to, BTEM-PPV(Poly(2,5-bis(1,4,7,10-tetraoxaundecyl)-1,4-phenylenevinylene),Poly(3-decyloxythiophene), CuPc (copper phthalocyanine), NPD(4,4′-bis(N-(1-napthyl)phenylamino)biphenyl), pentacene, tetracene, andthe like. The suitable organic semiconductors for the electron acceptorlayer 230A include, but are not limited to, evaporated C₆₀, [84]PCBM([6,6]-Phenyl C₈₄ butyric acid methyl ester), F16-CuPc, PTCBI (3,4,9,10perylenetetracarboxylic bisbenzimidazole), PTCDA (3,4,9,10perylene-tetracarboxylic dianhydride), orPoly(benzimidazobenzophenanthroline), TCNQ(7,7,8,8-tetracyanoquinodimethane), F4-TCNQ(tetrafluorotetracyanoquinodimethane), and the like.

Referring to FIG. 2B, the photovoltaic device embodiment 200B comprisesa conducting anode layer 210B and an electron donor layer 220B formedabove the anode layer 210B. A bulk heterojunction layer 230B comprisingpolymer wrapped carbon nanotubes is disposed within an organic electronacceptor material formed above the donor layer 220B, and a conductingcathode layer 250B is formed above the bulk heterojunction layer 230B.The bulk heterojunction layer 230B and the electron donor layer 220Bform the photoactive region 222B of the device 200B.

The suitable organic semiconductor acceptor materials for forming thebulk heterojunction layer 230B include, but are not limited to,evaporated C₆₀, [84]PCBM ([6,6]-Phenyl C₈₄ butyric acid methyl ester),F16-CuPc, PTCBI (3,4,9,10 perylenetetracarboxylic bisbenzimidazole),PTCDA (3,4,9,10 perylene-tetracarboxylic dianhydride), orPoly(benzimidazobenzophenanthroline), TCNQ(7,7,8,8-tetracyanoquinodimethane), F4-TCNQ(tetrafluorotetracyanoquinodimethane), and the like. The suitableorganic semiconductors for the electron donor layer 220B include, butare not limited to, BTEM-PPV(Poly(2,5-bis(1,4,7,10-tetraoxaundecyl)-1,4-phenylenevinylene),Poly(3-decyloxythiophene), CuPc (copper phthalocyanine), NPD(4,4′-bis(N-(1-napthyl)phenylamino)biphenyl), pentacene, tetracene, andthe like.

According to another embodiment 200C illustrated in FIG. 2C, both of theelectron acceptor layer 230C and the electron donor layer 220C in thephotoactive region 222C can be bulk heterojunctions comprisingpolymer-wrapped carbon nanotubes and the respective acceptor-type ordonor-type organic semiconductor materials.

In FIG. 2D, a bulk heterojunction PV device 200D according to anotherembodiment is shown. The device 200D comprises a conducting anode layer210D, a conducting cathode layer 250D and a bulk heterojunction layer220D provided between and electrically connected to the two electrodes.The bulk heterojunction layer 220D comprises polymer-wrapped carbonnanotubes disposed within an organic semiconductor material that can beeither an organic electron acceptor or electron donor materialsdisclosed herein. In this embodiment, the bulk heterojunction layer 220Dforms the photoactive region of the device 200D. Optionally, one or moreexciton blocking layers can be provided in the device. An excitonblocking layer 215D can be provided between the anode layer 210D and thebulk heterojunction layer 220D. Another exciton blocking layer 240D canbe provided between the cathode layer 250D and the bulk heterojunctionlayer 220D either in conjunction with the first exciton blocking layer215D or independent of the first exciton blocking layer 215D.

In the devices of 200A, 200B, 200C and 200D, preferably, thepolymer-wrapped carbon nanotubes are substantially semiconductingPW-SWNTs. An optional exciton blocking layer 240A, 240B, 240C can beprovided between the photoactive regions 222A, 222B, 222C and thecathode layers 250A, 250B, 250C, respectively. Additionally, an optionalexciton blocking layer 215A, 215B, 215C can be provided between thephotoactive regions 222A, 222B, 222C and the anode layers 210A, 210B,210C, respectively. An anode-smoothing layer may also be situatedbetween the anode and the donor.

FIGS. 3A-3B show additional hybrid planar-bulk heterojunctionembodiments for a photovoltaic device 300A and 300B. Referring to FIG.3A, the photovoltaic device embodiment 300A comprises a conducting anodelayer 310A, a thin film of polymer-wrapped carbon nanotubes as anelectron donor layer 320A formed above the anode layer 310A, and a bulkheterojunction layer 325A comprising polymer-wrapped carbon nanotubesdisposed within an organic electron acceptor material formed above thedonor layer 320A. An electron acceptor layer 330A is formed above thebulk heterojunction layer 325A and a conducting cathode layer 350A isformed above the acceptor layer 330A. The electron donor layer 320A, thebulk heterojunction layer 325A and the electron acceptor layer 330A formthe photoactive region 322A of the device 300A.

The suitable organic semiconductor acceptor materials for forming thebulk heterojunction layer 325A and the electron acceptor layer 330A arethe same as those discussed in conjunction with the embodiment of FIG.1A.

Referring to FIG. 3B, the photovoltaic device embodiment 300B comprisesa conducting anode layer 310B, an electron donor layer 320B formed abovethe anode layer 310B and a bulk heterojunction layer 325B comprisingpolymer-wrapped carbon nanotubes disposed within an organic electrondonor material formed above the donor layer 320B. A thin film ofpolymer-wrapped carbon nanotubes as an electron acceptor layer 330B isformed above the bulk heterojunction layer 325B and a conducting cathodelayer 350B is formed above the bulk heterojunction layer 325B. Theelectron donor layer 320B, the bulk heterojunction layer 325B and theelectron acceptor layer 330B form the photoactive region 322B of thedevice 300B.

As in the embodiment 300A, the suitable organic semiconductor acceptormaterials for forming the bulk heterojunction layer 325B are the same asthose discussed in conjunction with the embodiment of FIG. 1A. Thepossible organic semiconductor materials for the electron donor layer320B are same as those discussed in conjunction with the embodiment ofFIG. 1B.

In both embodiments 300A and 300B, the bulk heterojunction layers 325Aand 325B can be formed by depositing a mixed film of both organicsemiconductor material and polymer-wrapped carbon nanotubes, or by vapordeposition of an organic semiconductor onto a thin mat ofpolymer-wrapped carbon nanotubes. This bulk heterojunction layer maythen be sandwiched between a layer of polymer-wrapped carbon nanotubes(320A, 320B) and a layer of organic semiconductor (330A, 330B).

In both embodiments 300A and 300B, preferably, the polymer-wrappedcarbon nanotubes are substantially semiconducting PW-SWNTs. An optionalexciton blocking layer 340A, 340B can be connected between thephotoactive regions 322A, 322B and the cathode layers 350A, 350B,respectively. Additionally, an optional exciton blocking layer 315A,315B can be connected between the photoactive regions 322A, 322B and theanode layers 310A, 310B, respectively. An anode-smoothing layer may alsobe situated between the anode and the donor.

The small molecule organic semiconductors discussed herein can bedeposited by vacuum thermal evaporation (VTE), organic vapor phasedeposition (OVPD). or via solution-based processing methods. Dependingon the background growth pressure, substrate temperature, growth rate,the molecular structure of the organic semiconductor, and the roughnessof the substrate, various morphologies and degrees of crystalline ordercan be obtained, which influence charge transport and interfacialmorphology. In the instance in which organic semiconductors aredeposited directly on top of percolating networks of CNTs, the inherentroughness of the CNT network may be used to cause roughness-inducedcrystallization or crystalline growth in order to improve devicecharacteristics.

One technique for sorting carbon nanotubes by their band gap, diameter,and electronic-type that is currently amenable to the fabrication oforganic semiconductor-semiconducting CNT heterojunction photovoltaicsolar cells is density gradient ultracentrifugation (DGU) (M. S. Arnold,A. A. Green, J. F. Hulvat et al., Nature Nanotech. 1(1), 60 (2006); M.S. Arnold, S. I. Stupp, and M. C. Hersam, Nano Letters 5 (4), 713(2005)). Using DGU, bulk samples (gram scale) of up to 99% singleelectronic type (either semiconducting or metallic) can be readilyproduced. Furthermore, DGU can be used to sort SWNTs by their diameter,optical band gap, and electrical band gap as well.

Incorporation of a network of nanotubes into a matrix can be carried outby several methods known in the art, including but not limited to vapordeposition of the matrix material and spin-casting of polymer-nanotubeblends. (See for example U.S. Pat. No. 7,341,774 the content of which isincorporated herein by reference, and references therein.)

As discussed above, the properties of carbon nanotubes are influenced bythe diameter of the tube and its chirality. This is illustrated in FIGS.11A-11D where the current-voltage characteristics and spectrallyresolved photoresponse for a PW-CNT/C₆₀ heterojunction diode 400 of FIG.11A are shown. The diode 400 comprises an ITO anode layer 410, a PW-CNTlayer 420, a C₆₀ acceptor layer 430, and optional BCP electron blockinglayer 440 and a Ag cathode 450. The thickness of the layers are providedin FIG. 11A. The PW-CNT is wrapped with MDMO-PPV polymer at 1:1 ratio byweight and the PW-CNT/C₆₀ interface forms the heterojunction. The diode400 has dark current rectification ratios >10³ at ±1V (see FIG. 11B),which is particularly remarkable given that the PW-CNT layer 420consists of a high density of metallic tubes whose presence would beexpected to result in large shunt currents. The absence of suchparasitic effects from the metal tubes suggests that they are, indeed,electrically and energetically isolated from the semiconducting tubes bythe wrapped polymer.

Referring to FIG. 11B, a fit to the forward bias current-voltagecharacteristics (solid line) follows the ideal diode equation with anideality factor of 2.0 and a specific series resistance of 0.99 Ω-cm².Here, the ideality factor ˜2 suggests that carrier recombination is thedominant source of dark current, which is again remarkable consideringthe high density of metallic tubes which should lead to significantshunt currents (and hence resistance-limited transport).

The near-IR responsivities of the diode at 0 V and −0.7 V are comparedin FIG. 11C. Due to the diametric heterogeneity of the nanotubes,photoactive response is observed over a wide range from both E₁₁(λ≈900-1450 nm) and E₂₂ (λ≈550-900 nm) absorption features, with thepeak polymer response at λ=500 nm. The chirality indices (n,m) of thenanotube responsible for each absorption feature are labeled in the FIG.11C, as are the absorption regions of the polymer and small moleculeconstituents of the device. The very broad spectral coverage is a directresult of the diametric polydispersity of the SWNTs which collectivelycover the spectrum from 550 nm to 1600 nm.

Referring to FIG. 11C, the diode responsivity at λ=1155 nm at a bias of0 V and −0.7 V was 12 mA/W and 21 mA/W, respectively, corresponding to apeak EQE=2.3%. At λ=1300 nm, the detector responsivity was 11 and 21mA/W (EQE=2.0%), respectively, whereas the response of devices lackingCNTs at these wavelengths was not measurable (<0.1 μA/W). The IQE of theSWNTs in the near-IR (FIG. 11D) was >20% between λ=1000 and λ=1400,suggesting that SWNT-based devices with much higher EQE should beachievable.

Examples I. Materials

The method of Arnold et al., Nature Nanotech, 1:60-65 (2006) is used toisolate semiconducting CNTs using density gradient ultracentrifugation.A commercially-available CNT powder is suspended in water with a 1:4mixture of sodium dodecyl sulfate and sodium cholate (2% surfactant) byultrasonic treatment. The nanotube suspension is then loaded onto aniodixanol linear density gradient and centrifuged to sort the nanotubesby buoyant density. After fractionation of the density gradient, theiodixanol is removed by dialysis in surfactant solutions. Suitableorganic semiconductors are well-known in the art, and are commerciallyavailable from a number of suppliers.

II. Planar Heterojunction with Near-IR Sensitivity

Raw HiPCO single-walled carbon nanotubes (Carbon Nanotechnologies Inc.)(10 mg) were mixed with 10 ml of 2% (w/v) sodium cholate (Sigma-Aldrich,995) in water. The mixture was homogenized in an ultrasonic bath for 15minutes using a horn probe ultrasonicator. Coarse aggregates and largebundles of single-walled carbon nanotubes were then removed viaultracentrifugation (15,000 g, 12 hours). An aliquot of the resultingsuspension (100 μl) was filtered via vacuum filtration on an Al₂O₃membrane (0.02 μm pores, Whatman Inc.). The nanotube film was thentransferred to a planar PDMS stamp by pressing the PDMS into themembrane with finger pressure. The film was then stamped (1000 N-cm-2,60 s, room temperature, ambient atmosphere) onto a substrate consistingof 100 nm of PTCDA (3,4,9,10-perylene tetracarboxylic dianhydride) onAg-coated ITO (indium tin oxide). The PTCDA and Ag had been evaporatedvia thermal evaporation in a 1E−7 torr vacuum at a rate of 0.15 nm/s.Testing was done in ambient, and a xenon lamp plus an AM1.5G filter wasutilized to approximate the solar spectrum. A calibrated photodiode wasutilized to determine the light intensity.

The current density-voltage curve of the resulting device (FIG. 4) wasobtained by pressing a gold contact pad onto the surface of the SWNTnetwork and applying a voltage. The potential in the x-axis of FIG. 5denotes the potential of the carbon nanotube film with respect to thatof the ITO/Ag electrode. In the dark, the device exhibited typical diodebehavior, but when illuminated with simulated near-IR solar radiation(AMI 0.5 G spectrum, filtered through a dielectric long-pass filter witha 950 nm cut-off) a photoelectric effect was observed (FIG. 5).

IV. Bulk Heterojunctions

In one embodiment, a layer of substantially semiconducting or mixedSWNTs is transferred by PDMS stamping onto a transparent anode. Asuspension of substantially semiconducting SWNTs in a solution of anorganic acceptor is spin-cast onto the SWNT layer. An acceptor layer,and an optional electron transport and/or exciton blocking layer, arethen deposited, followed by a cathode layer.

In another embodiment, an organic donor layer is deposited onto an anodesubstrate, and a suspension of substantially semiconducting SWNTs in asolution of an organic acceptor is spin-cast onto the donor layer. Alayer of substantially semiconducting or mixed SWNTs is applied by PDMSstamping, followed by deposition of an optional electron transportand/or exciton blocking layer. A cathode layer is then deposited.

As discussed above, excitons in carbon nanotubes can be split byinterfacing them with the organic acceptor, evaporated C₆₀, and internalquantum efficiency as large as about 35% at 1200 nm has been observed.(See FIG. 9D). It is expected that the spectral range of the carbonnanotube/organic hybrid photovoltaic devices can further extend into thenear-infrared by using carbon nanotubes with larger diameter.

While the present invention is described with respect to particularexamples and preferred embodiments, it is understood that the presentinvention is not limited to these examples and embodiments. The presentinvention as claimed therefore includes variations from the particularexamples and preferred embodiments described herein, as will be apparentto one of skill in the art.

1. A device comprising: a first electrode; a second electrode; and aphotoactive region disposed between and electrically connected to thefirst electrode and the second electrode, wherein the photoactive regioncomprises an organic semiconductor material and photoactivepolymer-wrapped carbon nanotubes disposed therein whereby the organicsemiconductor material and the photoactive polymer-wrapped carbonnanotubes form a bulk heterojunction.
 2. The device of claim 1, whereinthe polymer-wrapped carbon nanotubes are substantially semiconductingpolymer-wrapped single-wall carbon nanotubes.
 3. The device of claim 2,wherein the polymer-wrapped single-wall carbon nanotubes are wrappedwith a photoactive polymer.
 4. The device of claim 2, wherein thepolymer-wrapped single-wall carbon nanotubes create excitons uponabsorption of light in the range of about 400 nm to 1400 nm.
 5. Thedevice of claim 1, wherein the bulk heterojunction layer comprisessubstantially semiconducting polymer-wrapped carbon nanotubes disposedwithin an electron acceptor type organic semiconductor material selectedfrom one of evaporated C₆₀, [84]PCBM ([6,6]-Phenyl C₈₄ butyric acidmethyl ester), F16-CuPc, PTCBI, PTCDA,Poly(benzimidazobenzophenanthroline), TCNQ(7,7,8,8-tetracyanoquinodimethane), and F4-TCNQ(tetrafluorotetracyanoquinodimethane).
 6. The device of claim 1, whereinthe bulk heterojunction layer comprises substantially semiconductingpolymer-wrapped carbon nanotubes disposed within an electron donor typeorganic semiconductor material selected from one of BTEM-PPV(Poly(2,5-bis(1,4,7,10-tetraoxaundecyl)-1,4-phenylenevinylene),Poly(3-decyloxythiophene), CuPc (copper phthalocyanine), NPD(4,4′-bis(N-(1-napthyl)phenylamino)biphenyl), pentacene, and tetracene.7. A device comprising: a first electrode; a second electrode; and aphotoactive region disposed between and electrically connected to thefirst electrode and the second electrode, the photoactive region furthercomprising: a donor layer formed above the first electrode; and anacceptor layer formed above the donor layer whereby the donor layer andthe acceptor layer form a donor-acceptor heterojunction, wherein eitherthe acceptor layer or the donor layer comprises photoactivepolymer-wrapped carbon nanotubes.
 8. The device of claim 7, wherein thepolymer-wrapped carbon nanotubes are substantially semiconductingpolymer-wrapped single-wall carbon nanotubes.
 9. The device of claim 8,wherein the polymer-wrapped single-wall carbon nanotubes are wrappedwith a photoactive polymer.
 10. The device of claim 8, wherein thepolymer-wrapped single-wall carbon nanotubes create excitons uponabsorption of light in the range of about 400 nm to 1400 nm.
 11. Thedevice of claim 7, wherein the donor layer comprises polymer-wrappedcarbon nanotubes and the acceptor layer comprises an organicsemiconductor material selected from one of evaporated C₆₀, [84]PCBM([6,6]-Phenyl C₈₄ butyric acid methyl ester), F16-CuPc, PTCBI, PTCDA,Poly(benzimidazobenzophenanthroline), TCNQ(7,7,8,8-tetracyanoquinodimethane), and F4-TCNQ(tetrafluorotetracyanoquinodimethane).
 12. The device of claim 11,wherein the polymer-wrapped carbon nanotubes are substantiallysemiconducting polymer-wrapped single-wall carbon nanotubes.
 13. Thedevice of claim 7, wherein the acceptor layer comprises polymer-wrappedcarbon nanotubes and the donor layer comprises an organic semiconductormaterial selected from one of BTEM-PPV(Poly(2,5-bis(1,4,7,10-tetraoxaundecyl)-1,4-phenylenevinylene),Poly(3-decyloxythiophene), CuPc (copper phthalocyanine), NPD(4,4′-bis(N-(1-napthyl)phenylamino)biphenyl), pentacene, and tetracene.14. The device of claim 13, wherein the polymer-wrapped carbon nanotubesare substantially semiconducting polymer-wrapped single-wall carbonnanotubes.
 15. The device of claim 7, further comprising an excitonblocking layer provided between the acceptor layer and the secondelectrode.
 16. The device of claim 7, further comprising an excitonblocking layer provided between the donor layer and the first electrode.17. A device comprising: a first electrode; a second electrode; aphotoactive region disposed between and electrically connected to thefirst electrode and the second electrode, the photoactive region furthercomprising: a donor layer formed above the first electrode; and anacceptor layer formed above the donor layer, wherein at least one of theacceptor layer or the donor layer comprises photoactive polymer-wrappedcarbon nanotubes disposed therein.
 18. The device of claim 17, whereinthe polymer-wrapped carbon nanotubes are substantially semiconductingpolymer-wrapped single-wall carbon nanotubes.
 19. The device of claim18, wherein the polymer-wrapped single-wall carbon nanotubes are wrappedwith a photoactive polymer.
 20. The device of claim 18, wherein thepolymer-wrapped single-wall carbon nanotubes create excitons uponabsorption of light in the range of about 400 nm to 1400 nm.
 21. Thedevice of claim 17, further comprising an exciton blocking layerprovided between the acceptor layer and the second electrode.
 22. Thedevice of claim 17, further comprising an exciton blocking layerprovided between the donor layer and the first electrode.
 23. The deviceof claim 17, wherein the donor layer comprises substantiallysemiconducting polymer-wrapped carbon nanotubes disposed within a donortype organic semiconductor material selected from one of BTEM-PPV(Poly(2,5-bis(1,4,7,10-tetraoxaundecyl)-1,4-phenylenevinylene),Poly(3-decyloxythiophene), CuPc (copper phthalocyanine), NPD(4,4′-bis(N-(1-napthyl)phenylamino)biphenyl), pentacene, and tetracene.24. The device of claim 23, wherein the polymer-wrapped carbon nanotubesare substantially semiconducting polymer-wrapped single-wall carbonnanotubes.
 25. The device of claim 24, wherein the polymer-wrappedsingle-wall carbon nanotubes are wrapped with a photoactive polymer. 26.The device of claim 17, wherein the acceptor layer comprisessubstantially semiconducting polymer-wrapped carbon nanotubes disposedwithin an acceptor type organic semiconductor material selected from oneof evaporated C₆₀, [84]PCBM ([6,6]-Phenyl C₈₄ butyric acid methylester), F16-CuPc, PTCBI, PTCDA, Poly(benzimidazobenzophenanthroline),TCNQ (7,7,8,8-tetracyanoquinodimethane), and F4-TCNQ(tetrafluorotetracyanoquinodimethane).
 27. The device of claim 26,wherein the polymer-wrapped carbon nanotubes are substantiallysemiconducting polymer-wrapped single-wall carbon nanotubes.
 28. Thedevice of claim 27, wherein the polymer-wrapped single-wall carbonnanotubes are wrapped with a photoactive polymer.
 29. The device ofclaim 17, wherein the donor layer comprises substantially semiconductingpolymer-wrapped carbon nanotubes disposed within a donor type organicsemiconductor material selected from one of BTEM-PPV(Poly(2,5-bis(1,4,7,10-tetraoxaundecyl)-1,4-phenylenevinylene),Poly(3-decyloxythiophene), CuPc (copper phthalocyanine), NPD(4,4′-bis(N-(1-napthyl)phenylamino)biphenyl), pentacene, and tetracene;and the acceptor layer comprises substantially semiconductingpolymer-wrapped carbon nanotubes disposed within an acceptor typeorganic semiconductor material selected from one of evaporated C₆₀,[84]PCBM ([6,6]-Phenyl C₈₄ butyric acid methyl ester), F16-CuPc having aLUMO of −4.4 eV, PTCBI, PTCDA, Poly(benzimidazobenzophenanthroline),TCNQ (7,7,8,8-tetracyanoquinodimethane), and F4-TCNQ(tetrafluorotetracyanoquinodimethane).
 30. The device of claim 29,wherein the polymer-wrapped carbon nanotubes are substantiallysemiconducting polymer-wrapped single-wall carbon nanotubes.
 31. Thedevice of claim 30, wherein the polymer-wrapped single-wall carbonnanotubes are wrapped with a photoactive polymer.
 32. A photovoltaicdevice comprising: a first electrode; a second electrode; and aphotoactive region disposed between and electrically connected to thefirst electrode and the second electrode, the photoactive region furthercomprising: a donor layer formed above the first electrode; a bulkheterojunction layer formed above the donor layer, wherein the bulkheterojunction comprises photoactive polymer-wrapped carbon nanotubesdisposed within an organic semiconductor material; and an acceptor layerformed above the bulk heterojunction layer, wherein either the acceptorlayer or the donor layer comprises photoactive polymer-wrapped carbonnanotubes.
 33. The device of claim 32, wherein the organic semiconductormaterial in the bulk heterojunction layer is an electron acceptor andthe donor layer comprises photoactive polymer-wrapped carbon nanotubes.34. The device of claim 33, wherein the bulk heterojunction layercomprises substantially semiconducting polymer-wrapped carbon nanotubesdisposed within an electron acceptor type organic semiconductor materialselected from one of evaporated C₆₀, [84]PCBM ([6,6]-Phenyl C₈₄ butyricacid methyl ester), F16-CuPc, PTCBI, PTCDA,Poly(benzimidazobenzophenanthroline), TCNQ(7,7,8,8-tetracyanoquinodimethane), and F4-TCNQ(tetrafluorotetracyanoquinodimethane).
 35. The device of claim 34,wherein the polymer-wrapped carbon nanotubes are substantiallysemiconducting polymer-wrapped single-wall carbon nanotubes.
 36. Thedevice of claim 35, wherein the polymer-wrapped single-wall carbonnanotubes are wrapped with a photoactive polymer.
 37. The device ofclaim 33, further comprising an exciton blocking layer provided betweenthe acceptor layer and the second electrode.
 38. The device of claim 33,further comprising an exciton blocking layer provided between the donorlayer and the first electrode.
 39. The device of claim 32, wherein theorganic semiconductor material in the bulk heterojunction layer is anelectron donor and the acceptor layer comprises photoactivepolymer-wrapped carbon nanotubes.
 40. The device of claim 39, whereinthe bulk heterojunction layer comprises substantially semiconductingpolymer-wrapped carbon nanotubes disposed within an electron donor typeorganic semiconductor material selected from one of BTEM-PPV(Poly(2,5-bis(1,4,7,10-tetraoxaundecyl)-1,4-phenylenevinylene),Poly(3-decyloxythiophene), CuPc (copper phthalocyanine), NPD(4,4′-bis(N-(1-napthyl)phenylamino)biphenyl), pentacene, and tetracene.41. The device of claim 40, wherein the polymer-wrapped carbon nanotubesare substantially semiconducting polymer-wrapped single-wall carbonnanotubes.
 42. The device of claim 41, wherein the polymer-wrappedsingle-wall carbon nanotubes are wrapped with a photoactive polymer. 43.The device of claim 39, further comprising an exciton blocking layerprovided between the acceptor layer and the second electrode.
 44. Thedevice of claim 39, further comprising an exciton blocking layerprovided between the donor layer and the first electrode.